Exhaust gas diffuser of a gas turbine

ABSTRACT

An exhaust gas diffuser for a gas turbine includes a flow duct which widens towards a diffuser outlet. The flow duct includes an axially extending guide body, wherein the guide body, at least in an axial section of its longitudinal extent, has a plurality of recesses which are distributed over the circumference and arranged between guide elements. Each guide element extends along a spiral line along the longitudinal extent of the guide body. Further, a gas turbine plant with such an exhaust gas diffuser is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2011/056210 filed Apr. 19, 2011, and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent Application No. 10004244.9 EP filed Apr. 21, 2010. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to an exhaust gas diffuser for a gas turbine,having a flow duct which widens towards the diffuser outlet and in thecenter of which provision is made for an axially extending guide body.

BACKGROUND OF INVENTION

Such an exhaust gas diffuser serves for decelerating the exhaust gasflow of a gas turbine and for recuperating at least some of the dynamicpressure. In order to disclose an exhaust gas diffuser with acomparatively large opening angle of 10° and more, it is known from DE198 05 115 A1, for example, to provide an axially extending guide bodyin the center of the diffuser duct. By using the guide body, the exhaustgas diffuser is designed as an annular diffuser. As a result, largerregions of backflow zones aft of the gas turbine hub are avoided, whichhas an advantageous effect upon the efficiency of the exhaust gasdiffuser. It is disadvantageous, however, that the guide body iscomparatively long and therefore has to be supported by additionalstruts on account of its length. In addition, the aerodynamic influencesof the support struts are not taken into account.

Furthermore, an exhaust gas diffuser of a gas turbine with a centraldrum body, on the generated surface of which provision is made for amultiplicity of blades which are arranged in a rotation-resistantmanner, is known from U.S. Pat. No. 5,791,136. The blades serve forhomogenizing the exhaust gas flow of the gas turbine in the event of asmall pressure drop.

SUMMARY OF INVENTION

It is an object to provide a space-saving exhaust gas diffuser for a gasturbine which for achieving an efficiency of the gas turbine which is ashigh as possible enables an improvement of the pressure recuperation.Also, the exhaust gas diffuser is to enable homogenization of thediffuser outflow so that in a heat recovery steam generator, which isconnected downstream of the gas turbine, an inflow, which is ashomogeneous as possible, of the hot exhaust gas from the gas turbine iseffected.

This object is achieved by an exhaust gas diffuser as claimed in theclaims. To this end, the guide body of the generic-type exhaust gasdiffuser, at least in an axial section of its longitudinal extent, has anumber of recesses which are distributed over the circumference andarranged between the guide elements, wherein each guide element extendsalong a spiral line.

The invention is based on the knowledge that the secondary flows whichare initiated on the support struts of the rear bearing star in theevent of swirled inflow are to be utilized in order to intensify themixing-through aft of the hub end and so to further shorten the lengthof backflow zones aft of the hub. For this purpose, the guide body ofthe exhaust gas diffuser, which is designed as an essentially conical orcylindrical body, has recesses and guide elements which are distributedover the circumference in order to guide and to intensify the secondaryflows in an optimum manner. The secondary flows guide fluid from thepressure side of the support struts onto spiral-like paths. By usingrecesses and guide elements, the secondary flows can therefore getcloser to the center axis of the diffuser and the support body can be ofa shorter design than in the case of the prior art without itsefficiency being reduced in the process. As a result of the shortenedlength of the support body, this can also be of free-standing design.The particular advantage of the invention lies in the utilization of thesecondary flows which are generated on the pitched and/or profiledsupport struts in order to guide fluid still closer to the hub axis withthe aid of hub-side recesses in order to thus further reduce the sizeand especially the length of the backflow zone aft of the hub. Thisleads to further increased efficiency of the exhaust gas diffuser.

As a result of the pitched orientation of the guide elements (pitch) inrelation to the center axis, both an exceptionally efficient andspace-saving construction for de-swirling of the flow is disclosed. Thepitch of the spiral line is comparatively large in this case so that thespiral lines are only slightly inclined in relation to the center axisof the exhaust gas diffuser. The pitch can also become smaller along itsextent. Preferably, it increases, however, in order to obtain an outflowfrom the exhaust gas diffuser which has as little swirl as possible.This arrangement is particularly advantageous in the case of an axialexhaust gas diffuser in which the number of guide elements is the sameas a number of support struts which are provided in the exhaust gasdiffuser or directly upstream thereof for the turbine-side support ofthe gas turbine rotor.

In an advantageous embodiment, the guide body has a cross-sectionalcontour which is arranged perpendicularly to the center axis of theexhaust gas diffuser and which is in the shape of a star, the points ofwhich are formed by the respective guide elements. By means of thestar-shaped cross-sectional contour, significantly greater rigidity ofthe inner wall of the exhaust gas diffuser, now formed as an annulardiffuser, can be achieved. The tendency of the guide body towardsvibrating on account of the flow is significantly reduced as a result.Consequently, it is also possible to design the guide body in a way inwhich it is fastened on only one side so that its end facing thediffuser outlet overhangs in the exhaust gas diffuser in a freelysuspended manner. An aerodynamically disturbing and cost-increasingoutlet-side support of the guide body can consequently be avoided onaccount of the greater rigidity of the guide body.

In an expedient development, each guide element has an outer tip whichprojects radially outward from the center axis of the exhaust gasdiffuser, the shortest distance of which to the center axis (that is tosay in the cross section arranged perpendicularly to the center axis)reduces with decreasing axial distance to the diffuser outlet. In thiscase, each recess has a radially inward disposed base, the shortestdistance of which to the center axis (that is to say in the crosssection arranged perpendicularly to the center axis) also reduces withdecreasing axial distance to the diffuser outlet. As a result of this, awidening of the flow cross section of the exhaust gas diffuser is alsoachieved on the hub side, which contributes to a further shortening ofthe overall axial length of the exhaust gas diffuser. The flow crosssection increase must therefore be achieved not only via an increase ofthe diameter of the outer annular wall of the exhaust gas diffuser. Thismeasure also reduces the overall axial length of the exhaust gasdiffuser.

Each guide element—as seen in the flow direction of the exhaust gasdiffuser—is preferably arranged in an aligning extension of the supportstruts. Particularly as a result of this, it is possible to utilize thesecondary flows which are initiated on the support struts of the rearbearing star in the event of swirled inflow in order to intensify themixing-through aft of the hub end and to achieve improved pressurerecuperation. In particular, the secondary flow vortices along thepressure side of the support struts can then be directed in anaerodynamically especially favorable manner into the recess of the guidebody, which brings along only comparatively low aerodynamic losses. Inthis case, it can be provided that in that axial section of the exhaustgas diffuser in which the support struts are arranged, the guide body,between the struts, already has an inner wall contour in the shape of anarc in the circumferential direction, the shortest distance of which tothe center axis of the exhaust gas diffuser reduces with decreasingaxial distance to the diffuser outlet. In other words, not just theguide body, which is connected to the support struts, can be of conicaldesign but the inner wall of the annular exhaust gas diffuser in theaxial section of the support struts can also already be of conicaldesign, wherein its diameter also decreases to the outlet-side end ofthe exhaust gas diffuser. Especially in the case of a heat recoverysteam generator, which is connected downstream of the gas turbine on theexhaust gas side via the exhaust gas diffuser, a saving in material isachieved by shortening associated steam lines to the heat recovery steamgenerator or heat recovery boiler.

A corresponding heat recovery steam generator or boiler in horizontal orvertical style of construction can then be designed with a relativelysmall heating surface since as a result of a comparatively homogeneousor more uniform inflow this is utilized better than before. In this way,the pressure loss of the boiler on the exhaust gas side reduces, whichin turn results in an efficiency gain of the gas turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in more detail withreference to a drawing.

FIG. 1 shows an exhaust gas diffuser of a gas turbine in a partiallysectioned, partially perspective view with a guide body arranged in thecenter,

FIG. 2 shows the cross section through the guide body according to FIG.1 in a first sectional view, and

FIG. 3 shows the guide body according to FIG. 1 in a second section.

DETAILED DESCRIPTION OF INVENTION

Shown in FIG. 1 is an exhaust gas diffuser 10 for a gas turbine in alongitudinal section, wherein the bearing star 12, which is arranged inthe center of the exhaust gas diffuser 10, for the turbine-side supportof the gas turbine rotor and of the guide body 14 which is arranged onthe bearing star, is shown in perspective view. The exhaust gas diffuser10 has a center axis 16 lying in its center, which extends from aninlet-side end 18 to an outlet-side end 20. Provision is made in theexhaust gas diffuser 10 for a flow duct 22, the cross section of which,being perpendicular to the center axis, has an annular contour. The flowduct 22 is delimited in this case by a radially outward disposed outerwall 24. The guide body 14 is arranged in the center of the exhaust gasdiffuser 10, that is to say in the region of its center axis 16, and inthis case constitutes the radially inward disposed boundary of the flowduct 22. The guide body 14 is supported in this case by five supportstruts 26 which are distributed evenly or unevenly around thecircumference. In the axial section A of the exhaust gas diffuser 10, inwhich the support struts 26 are also arranged, the turbine-side end ofthe gas turbine rotor, which for reasons of clarity is not shown,terminates inside the guide body. Upstream of the section A, that is tosay towards the inlet-side end 18, the guide body 14 is of cylindricaldesign. It is provided that in the section A the guide body 14 alreadydoes not have a cylindrical inner wall contour between the supportstruts 26 but that recesses 30 which become larger (deeper) are alreadyprovided there. Downstream of the section A, the recesses 30, which aredistributed over the circumference of the guide body 14, are delimitedby guide elements 32 so that recesses 30 and guide elements 32 arearranged in an alternating manner in the tangential direction. Thenumber of guide elements 32 corresponds in this case to the number ofsupport struts 26, five guide elements 32 and support struts 26 beingprovided in each case in the depicted exemplary embodiment. A greater orlesser number of guide elements 32 and support struts 26 can also beprovided. If the support struts 26 are distributed unevenly over thecircumference, the recesses 30 and guide elements 32 are distributedcorrespondingly unevenly on the inlet side. On the outlet side, theireven distribution is preferred in any event. As is known, the supportstruts 26 are aerodynamically profiled. Therefore, the support struts 26have a prespecified outflow angle, which is also known from turbineblades and compressor blades, in the aerodynamically aligningcontinuation of which the guide elements 32, being inclined in relationto the center axis, are twisted towards the outlet-side end 20 of theexhaust gas diffuser 10. Each guide element 32 has an outwardlyprojecting tip 36, the outermost point of which has a shortest distanceto the center axis of the exhaust gas diffuser 10—for any chosen crosssection which is perpendicular to the center axis 16—which distancereduces with decreasing axial distance of the cross section to thediffuser outlet 20. The same applies to each recess. The shortestdistance of the radially inward disposed base 38 to the center axis 16reduces with decreasing axial distance to the diffuser outlet. As aresult, the guide body 14, downstream of its section A, altogethermaintains a conical appearance which leads to the inner cross-sectionalarea of the guide body, through which exhaust gas does not flow,steadily reducing. At the end of the guide body, only a comparativelysmall cross section therefore remains, which can lead to backflowregions which are of a comparatively shorter and weaker formaccordingly.

Shown in FIGS. 2 and 3 are the cross-sectional views, arrangedperpendicularly to the center axis 16, of the guide body 14 according tothe sections II and III from FIG. 1. In this case, identical featuresare provided with identical designations.

In all, the invention discloses an exhaust gas diffuser 10 for a gasturbine, having a flow duct 22 which widens towards the diffuser outlet20, in the center of which provision is made for an axially extendingguide body 14. In order to disclose a further shortened exhaust gasdiffuser 10, the guide body 14 of which has a particularly high rigidityand the tendency of which towards flow-induced vibrations is reduced, itis provided that the guide body 14, at least in an axial section of itslongitudinal extent, has a number of recesses 30 which are distributedover the circumference and arranged between guide elements 32.

1.-10. (canceled)
 11. An exhaust gas diffuser for a gas turbine,comprising: a flow duct which widens towards a diffuser outlet, the flowduct comprising an axially extending guide body, wherein the guide body,at least in an axial section of its longitudinal extent, has a pluralityof recesses which are distributed over the circumference and arrangedbetween guide elements, wherein each guide element extends along aspiral line along the longitudinal extent of the guide body.
 12. Theexhaust gas diffuser as claimed in claim 11, wherein the guide body hasa cross-sectional contour which is arranged perpendicularly to a centeraxis of the exhaust gas diffuser, wherein the cross-sectional contour isin the shape of a star, and wherein the points of the star are formed bythe guide elements.
 13. The exhaust gas diffuser as claimed in claim 11,wherein each guide element has a tip which projects radially outwardfrom a center axis of the exhaust gas diffuser, wherein the shortestdistance of the tip to the center axis of the exhaust gas diffuserreduces with decreasing axial distance to the diffuser outlet.
 14. Theexhaust gas diffuser as claimed in claim 11, wherein each recess has aradially inward disposed base, wherein the shortest distance of the baseto a center axis of the exhaust gas diffuser reduces with decreasingaxial distance to the diffuser outlet.
 15. The exhaust gas diffuser asclaimed in claim 11, wherein an axial section of the guide body withrecesses and guide elements extends over an entire axial extent of theguide body.
 16. The exhaust gas diffuser as claimed in claim 11, whereinthe plurality of guide elements corresponds to a number of supportstruts which are provided for the turbine-side support of the gasturbine rotor.
 17. The exhaust gas diffuser as claimed in claim 16,wherein each guide element, as seen in a through flow direction of theexhaust gas diffuser, is arranged in an aligning extension of thesupport struts.
 18. The exhaust gas diffuser as claimed in claim 16,wherein in an axial section of the support struts provision is made foran inner wall contour, which is in the shape of an arc in acircumferential direction, between the support struts, and wherein theshortest distance of the inner wall contour to a center axis of theexhaust gas diffuser reduces with decreasing axial distance to thediffuser outlet.
 19. A gas turbine plant, comprising: an exhaust gasdiffuser, comprising: a flow duct which widens towards a diffuseroutlet, the flow duct comprising an axially extending guide body,wherein the guide body, at least in an axial section of its longitudinalextent, has a plurality of recesses which are distributed over thecircumference and arranged between guide elements, wherein each guideelement extends along a spiral line along the longitudinal extent of theguide body.
 20. The gas turbine plant as claimed in claim 19, furthercomprising a heat recovery steam generator which is connected downstreamof the exhaust gas diffuser.